The present invention relates to volume measuring sensors, and more particularly to a sensor for measuring the volume of air present in a container or a tank, thereby providing an indication of the volume of a liquid, powder or solid occupying the remaining volume of the container.
Several factors have come to the fore in recent years to suggest that a new design for a fuel quantity gauge is becoming a necessity. With the advent of consumers desiring smaller automobiles, designers would like the flexibility of employing convoluted fuel tanks to achieve space efficiency. This will necessitate a change from the simple fuel level gauges in use today.
Consumers, having voiced their desire for longer and more inclusive warranties, are getting such from automobile manufactures. As a consequence, manufacturers are looking for methods to lessen repair costs wherever possible. Presently if an automobiles is brought in for repair because of a defective fuel gauge, the entire fuel tank is replaced. The cost of dissecting the old fuel tank and repairing the gauge is prohibitive. Manufacturers would like externally mounted or easily removable fuel sensors so that the good fuel tank would not have to be discarded, thereby reducing warranty repair costs.
An unlevel vehicle and/or fuel sloshing contribute sources of error to fuel level gauges. Considering the increasing amount of stops and starts for today's commuting driver, the fuel sloshing could render the fuel gauge inaccurate for a large fraction of the time. This enhances the need for a level and sloshing insensitive fuel quantity sensor.
The new dashboard displays can display a high degree of accuracy in their readouts. So much so, that now the limiting factor in the accuracy of reading the remaining fuel is no longer in the display but in the fuel quantity gauge itself. Car manufacturers would like more accurate fuel gauges.
These four compelling reasons indicate the definite need for a new or improved fuel quantity gauge. An accurate, externally-mounted or removable, fuel volume sensor would provide a solution to all of the problems mentioned above.
The conventional gauges have been used in the measuring of fuel for years. The automobile, with its relatively quiescent journey and limited elevation angle, typically employs the mechanical float sensor. This sensor detects level of fluid in the tank and is inexpensive.
Some airplanes use mechanical float sensors, but most use a cylindrical capacitive sensor. Whichever sensor is used, a matrix of these sensors (from 4 to 12, typically) is typically used within each fuel tank inside the plane. There are several separate fuel tanks within an airplane to take best advantage of this limited space available within a plane's wings and fuselage. This matrix of sensors and averaging electronics is required to allow some measure of accuracy during banking and climbing. The capacitive sensor is also more accurate than the mechanical float sensor and therefore there is less likelihood of running out of fuel. However, the capacitive sensor is more expensive than the mechanical float sensor, making a matrix of such sensors prohibitively expensive for use in automobiles.
A simple mechanical float fuel sensor consists of a float (which always rides at the level of the fuel) and vertical rails which constrain the float. For a reference, see E. W. Pike et al., "Investigation of Fuel Quantity Measuring Techniques," DTIC-AD712120, USAF-AMC Wright Patterson AFB, Ohio, June 1952.
This sensor produces either a changing voltage or current as the float moves up and down along the rails. There have been numerous advances in the mechanical float sensor. However, most mechanical float sensors tend to suffer from the following general disadvantages: (1) Mechanical float sensors require some electricity within the fuel tank. Although not a problem, this is still disadvantageous from a safety standpoint. (2) Mechanical mechanisms of any sort break down with much higher regularity than any other system having no moving parts. (3) Mechanical floats have lower accuracies than other fuel gauges available today. (4) Mechanical float gauges measure only fuel level. This is a disadvantage as fuel sloshing, inclining of the road, and the convoluted fuel tank shapes of today decrease the accuracy of fuel level as a measure of fuel quantity.
Another conventional fuel gauge is the capacitive array fuel gauge. This gauge has been used for years in the fuel tanks of both commercial and military aircraft. For references, see W. B. Engle and R. M. Owen, "Electrical and Physical Nature of Microbial Membranes Implicated in Aircraft Fuel Quantity Probe Malfunction," SAE-710439, National Air Transportation Meeting, Atlanta, Georgia, May 1971; J. Huddart, "An Alternative Approach to Fuel Gauging," SAE-790138, Society of Automotive Engineers, Detroit, Mich., February/March 1979; K. Suzuki, T. Tomoda, and S. Momoo, "A Highly Accurate Fuel Level Measuring System," SAE-871961, Passenger Car Meeting, Dearborn, Mich., October 1987; P. Weitz and D. Slade, "Effects of Anti-Static Additives on Aircraft Capacitance Fuel Gauging Systems," AFWAL Wright Patterson AFB, Ohio, Technical Report #AFWAL-TR-80-2058, June 1980. The main reason that this gauging technique is preferred is because it is employed in an array configuration. By using multiple sensors in a clever array-like arrangement an array gauge can average out any tilting of the fuel tank.
The basic capacitive fuel sensor consists of two coaxial cylindrical electrodes, where the fuel sits between the walls of each cylinder. As the fuel tank fills, the height of the fuel between the electrodes increases. The fuel acts as a dielectric medium, thus altering the overall capacitance of the cylindrical capacitor probe.
The probe can be treated as two parallel capacitors in parallel. The first capacitor is the lower half of the probe, having the fuel dielectric between the electrodes. The second capacitor is the upper part of the probe, having only the air gap between the electrodes. The fuel height is inversely proportional to the dielectric constant of the fuel and directly proportional to the measured capacitance. The disadvantages of this sensor are: (1) the capacitive sensor is an expensive sensor. Arrays of such sensors further increase the cost. (2) The capacitive sensor by nature uses electrical contact in the tank. (3) Microbial growth in the fuel tank has been shown to affect the accuracy of this sensor.
A fiber optic liquid level gauge is described in J. W. Berthold, "Fibre Optic Intensity Sensors," Photonics Spectra, 22(12), 125-138 (December 1988), and utilizes two fibers, a prism, an LED, and a detector. The fiber must be arranged so that the light enters the prism from the first fiber and bounces off of the bottom face at the critical angle for a glass/air interface. The second fiber is for receiving light reflected from the prism. The critical angle is that angle at which no transmitted wave is produced into the second medium. Snell's law governs the angle of the transmitted and reflected waves at an interface. If light in the prism strikes the face at the critical angle, then there will be no transmitted wave and all the light will be reflected into the second fiber. If this is used in a tank and the liquid level comes up to the prism, then the interface is now a glass/liquid one. The transmitted angle is then not 90.degree., and the transmitted wave exists. The reflected wave will now have less energy than in the previous case. This drop in the intensity of the reflected wave can then be sensed at the detector. Multiple fiber sensors, each of different length, can be employed to provide an incremental level capability. The disadvantages of the fiber optic fuel gauge are: 1) the sensor must be located inside the tank, 2) films can form on the prism and foul the sensor, 3) the fiber optic sensor is a discrete sensor, and 4) the fiber optic sensor is a level sensor only.
There are two techniques associated with another known fuel sensor, the Boyle's Law or pressure fuel quantity gauge. (For references, see: K. Wantanabe and Y. Takebayashi, "Volume measurement of liquid in a deformed tank," SAE-871964, Passenger car meeting, Dearborn, Mich., October, 1987; H. Garner and W. Howell, "Volumetric Fuel Quantity Gauge," U.S. patent application, NASA-CASE-Lar-13147-1, August 1984.) The first technique (the Beckman method) uses isothermal compression to measure the volume of the gas. Any isothermal (constant temperature) change in volume is accompanied by a change in pressure. Measuring this pressure change, as a piston which is connected to the system collapses its volume, yields a measure of the entire tank volume. This technique has one major drawback. It cannot work in a tank that has vent holes or leaks of any kind. Such leaks would not allow the pressure build up that is so critical to the measurement. A second method proposed by Wantanabe and Takebayashi, id., uses an adiabatic (no heat flow) process and a step function of pressure to determine the volume of the air in the tank. This method can deal with small, medium, and large holes in the tank. The effect of leaks in the tank do not alter the outcome of the gauge, they only modify the relaxation time and damping of the pressure pulse in the tank. By noticing the speed of decay of the pressure after the step response, the gas volume can be determined. The disadvantages of this system are (1) the system is bulky and heavy, (2) the adiabatic system requires more complex electronics, and (3) the pistons and valves involved together with the electronics cause this gauge to be very expensive compared to other automobile fuel gauges.
A nuclear decay gauge consists of a nuclear decay source such as Americium (Am) or Krypton (Kr) gas (or an array of such sources) and a detector (such as a Geiger-Mueller tube or solid state radiation detector). For references, see K. V. Pearson, "Nucleonic Fuel Quantity Gauging System," Society of Automotive Engineers, Seattle, November, 1974; J. R. Webster, "Nucleonic Massmetric Instrumentation of Propellants for Aircraft," Air Force Flight Dynamics Laboratory, Wright Patterson Air Force Base, Ohio, Technical Report #AFFDL-TR-70-127, April, 1971; J. J. Singh, G. H. Mall, D. R. Sprinkle, and H. Chegini, "Feasibility of a Nuclear Gauge for Fuel Quantity Measurement Aboard Aircraft," NASA-TM-87706, August, 1985; and D. R. Sprinkle and C. Shen, "A method for monitoring the variability in nuclear absorption characteristics of aviation fuels," NASA-TM-4077, 1988. The source(s) are arranged so that the fuel lies between it and the detector(s). A simple system such as this consists of one source, a rectangular tank and one detector. The disadvantage of this system are (1) the detector and source are expensive, (2) the gauge can probably not be made removeable, and (3) this gauge yields a level measurement only.
The acoustic pipe resonant fuel gauge uses a simple pipe submerged within the fuel standing vertically within the tank, but open at the bottom to allow fuel to fill the tube. A speaker is used to drive standing waves within the open tube. The resonant frequency and therefore the standing wavelength within the tube determine the height of the volume of liquid in the tank. For reference, see B. D. Keller, C. R. Mayer, and B. Blanter, "Acoustic Fuel Quantity Measurement System," NADC-78187-60, NADC Warminster, Pa., February 1980. The resonant frequency of the pipe is determined by whether an odd integral number of 1/4 wavelengths will exactly fit the space between the speaker and the fuel level. The disadvantage of the acoustic pipe resonant fuel gauge is that it is a fuel level gauge only.